1. Field of the Invention
The present invention relates to a method of manufacturing a display device, and particularly relates to a display device which uses a thin film transistor in a pixel portion and a method of manufacturing the display device.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) with the use of a semiconductor thin film (with a thickness of several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors have been widely applied to electronic devices such as ICs and electro-optical devices, and their development especially as switching elements for image display devices has been accelerated. Channel regions of thin film transistors are formed of amorphous silicon or polycrystalline silicon.
As an alternative to these two kinds of semiconductors, TFTs whose channel regions are formed of microcrystalline silicon are also known (see Patent Document 1: Japanese Published Patent Application No. H4-242724 and Patent Document 2: U.S. Pat. No. 5,591,987). Microcrystalline silicon is formed by a plasma CVD method in a manner similar to amorphous silicon. For example, Patent Document 3 (Japanese Patent No. 3201492) has disclosed the invention, that is, a method of manufacturing a microcrystalline silicon film by a plasma CVD method with the use of a high frequency of a VHF (very high frequency) band of 30 MHz or more.
In general, in a case of forming a bottom-gate thin film transistor which uses a microcrystalline semiconductor film as a channel formation region, a semiconductor film with an amorphous structure is formed thinly at an interface with a gate insulating film. This thin semiconductor film with an amorphous structure might adversely affect electrical characteristics of the thin film transistor. In order to solve this problem, for example, Patent Document 4 (Japanese Published Patent Application No. H7-94749) has suggested a method in which a semiconductor film is formed at different speed in such a way that the speed is slow initially and is later increased.
Now, liquid crystal panels are manufactured as follows: a plurality of panels over a large-area glass substrate called a mother glass is processed, and then the panels are divided in accordance with the dimension of a screen of a TV or a personal computer. This is because the cost per panel can be reduced by taking plural panels out from one mother glass. In the market for liquid crystal TVs, the increase in screen size (panel size) and the decline in retail price have progressed rapidly. In order to increase the productivity in response to the increase in screen size and the decline in price, the enlargement of mother glasses has been promoted in these years.
Typical glass substrates around 1991 called the first generation were 300 mm×400 mm in size. After that, the mother glass has been enlarged in the second generation (400 mm×500 mm), the third generation (550 mm×650 mm), the fourth generation (730 mm×920 mm), the fifth generation (1000 mm×1200 mm), the sixth generation (2450 mm×1850 mm), the seventh generation (1870 mm×2200 mm), the eighth generation 2000 mm×2400 mm), the ninth generation (2450 mm×3050 mm), and the tenth generation (2850 mm×3050 mm).